Cupolas are widely used in foundries to melt pig iron, iron scrap and steel scrap or mixtures thereof. In order to operate a conventional cupola, a red hot bed of coke is established at its bottom. The coke bed is maintained at the desired temperature by supplying an air blast through tuyeres that direct the air at relatively low velocity into the bed. A charge comprising alternate layers of metal to be melted and coke is fed into the shaft of the cupola. Hot gases created by the exothermic reaction of the air blast with the coke bed flow upwards through the shaft of the cupola and heat the metal by convection sufficiently for a region of molten metal to be created immediately above the coke bed. The molten metal percolates through the coke bed and is superheated by radiation from the coke. From time to time molten metal is tapped off from the bottom of the cupola into a ladle for use in the foundry. Alternatively, the molten metal may be continuously tapped and collected in a suitable receiver. Although the coke in the bed is progressively consumed by the reaction with the oxygen component of the air blast, the coke layers in the charge will replenish the bed and the coke bed is maintained at adequate depths throughout the operation of the cupola. It is al so conventional to include within the charge limestone or other slag-forming agent, ferrosilicon or other suitable ferroalloys so as to improve the metallurgical properties of the metal during the melting operation.
A wide range of different variants of this basic method of operating a cupola are known. For example, the air blast can be provided without being preheated. Cupolas that operate in this way are known as cold-blast cupolas. Alternatively, the air blast can be preheated. Such cupolas are known as "hot blast" cupolas. If desired, the air blast may be enriched with oxygen so as typically to raise the oxygen concentration of the air by from 2 to 4% by volume. More preferably, the oxygen may be introduced into the coke bed in the form of high velocity jets through lances. The lances may be located below the tuyeres (see GB-A-914 904) or may project through the tuyeres themselves. (See GB-A-L 006 274). As disclosed in EP-A-56 644 the oxygen jets may each enter the cupola at above sonic velocity. All the variants described above that make use of oxygen offer two main advantages. First, they enable higher temperatures to be created within the cupola and thus enable the molten metal to be discharged at a higher temperature. Second, they enable the rate of melting metal to be increased.
It has been proposed in GB-A-L 500 511 to modify a conventional air blast cupola by adding to it oxy-fuel burners so as to provide additional heating to melt the metal. Accordingly, there is a reduced need for heat to be generated by the reaction between the air blast and the coke bed. As a result, the amount of coke in the charge can be reduced.
All the methods of operating cupolas described above suffer from a common disadvantage, namely that there is emitted from the top of the cupola a visible smoke or fume which is heavily laden with particles. Although it is possible to treat such smoke or fume to reduce its content of particles so as to render it less unsuitable for discharge to the atmosphere, the cost of so doing is high. There is therefore a growing demand for methods of operating cupolas which do not inevitably have associated therewith the production of a visible, particulate-laden fume.
In order to meet this demand there has been developed a cupola which uses neither an air blast nor coke. Instead, it employs air-fuel burners to melt the ferrous metal by convection heating, and a bed of ceramic balls to superheat the molten metal by radiant heat. The bed of ceramic balls is supported on a water-cooled grid. Immediately below the grid is a cavity into which the burners fire. The hot combustion gases ascend the furnace, heating the ceramic balls and melting the ferrous metal. The resulting molten metal falls through the ceramic balls and is superheated by heat radiated therefrom. There is thus no need to include any coke in the charge to the cupola, and provided that the ferrous metal in the charge is free of oil or other such contaminants, no visible fume is emitted. In practice, there have found to be a number of disadvantages associated with the operation of such cupolas. First, difficulties arise in producing molten metal at an adequate temperature. Moreover, the water-cooled grid tends to be damaged if excessive temperatures are created within the cupola. It has also been found that increased additions of ferrosilicon are required in order to ensure that a molten ferrous metal having a desired silicon content is given. Similarly, it is necessary to add carbon, typically in the form of graphite, to the molten metal to give a desired carbon content now that coke is no longer employed in the charge. Furthermore, the ceramic balls have a limited life as they tend to be eroded by the molten metal. There is therefore a need continuously to replace the balls, much in the same way as it is required in a conventional air blast cupola to include coke in the charge so as to replace the coke that is consumed by reaction with oxygen in the bed at the bottom of the cupola.
There is therefore a need for an alternative method of operating a cupola which does not of necessity entail the emission of large quantities of visible, particle-laden, fume from the furnace yet which facilitates the production of metal, particularly ferrous metal, at a temperature suitable for the direct casting of engineering iron without the need for an additional heating facility such as an electric duplexing furnace.